The disclosure relates generally to devices and methods for preventing a primary mud relief valve from incorrect opening. More particularly, the disclosure relates to devices and methods for preventing a primary relief valve in a mud system from incorrect opening.
Mud systems, as known from drilling rigs, normally include a mud pump, a pulsation dampener and a relief valve, the latter herein termed “primary relief valve”. The main pump is typically a triplex pump. This kind of pump delivers a flow rate which is far from constant and fluctuates much because of i) variable piston speed—the pump is crank shaft driven, ii) mud compressibility and iii) valve and fluid inertia. The pulsation dampener is therefore included to smoothen the flow rate and mitigate the resulting pressure fluctuations. The primary relief valve may be of a design known as a “pop-off valve” in the industry, or a rupture disk. Other types of relief valves are also known.
The primary relief valve is a safety valve that is designed to prevent excessive pressure and possible hazards in the case the pump pressure exceeds the certified pressure limit for the system. Normally the so-called trip pressure, at which the primary relief valve shall switch from a closed to an open state, is set slightly higher that the system pressure of typically 5000 psi (345 bar).
It is a well-known problem in the industry that the primary relief valve sometimes trips frequently even though the recorded pressure levels never reached the nominal trip pressure level.
Tripping of a primary relief valve represents a costly and highly undesired disruption of the drilling process, both because of the time it takes to refit a new rupture disk or to reset a resettable primary relief valve and because the well can be damaged during long periods of no circulation. To lower the risk for primary relief valve tripping one can therefore reduce maximum working pressure to say 80 per cent of the system pressure limit. This is also a costly solution because flow rate and pressure is often a limiting factor that can lead to slower drilling and even cause well stability problems. The opposite solution of increasing the nominal trip pressure to compensate for the dynamic effect is also a highly undesirable solution that may lead to damage in other parts of the mud system. It may even be illegal to raise the nominal relief valve pressure to more than a few per cent over the certified system pressure.